The \gamma -rays from \gamma -ray bursts ( GRBs ) are believed to be produced by internal shocks driven by small timescale , \sim 1 ms , variation in the GRB outflows , and a pair-production spectral cutoff is generally expected around the GeV range .
However , the observed optical flashes accompanying GRBs suggest that the delayed residual collisions due to large timescale variation continue to accelerate electrons .
We show here that the inverse-Compton ( IC ) scattering of the prompt \gamma -rays by these residual internal shock electrons leads to a high energy emission beyond the previously thought spectral cutoff , in agreement with the previous detections of GeV photons by EGRET in several GRBs in conjunction with MeV emission .
We expect a spectral break due to the transition from the primary to residual internal shock emission at the previously thought spectral cutoff , and expect systematic time delays of high energy photons relative to MeV emission , the discovery of which would provide stringent constraint on the outflow properties , but requires large enough collection of high energy photons by , e.g. , Fermi and AGILE satellites .

The recent Fermi-detected bright GRB 080916c unambiguously shows the shifting of the prompt emission toward later times as the photon energy increases .
The second-scale shifting at ¿100 MeV is much longer than the MeV variability time , as predicted in the residual collision model .
The observations imply that there should be emission above 70 GeV in the source frame , which may not be produced by primary internal shocks but by IC emission in residual collisions .
With the method involving time delays of high energy emission , the bulk Lorentz factor of GRB 080916c is determined to be \Gamma \sim 300 .